Portraiture is a cruel art. It does not matter if you wield a paintbrush or a camera. The eye that scrutinises is an eye that sees warts, wrinkles, twitches and scars. This ruthless revelation of weakness is at the brutally perceptive heart of Lucian Freud's paintings, unveiled in his first posthumous exhibition at the National Portrait Gallery this week. Freud is one the harshest of all great portrait painters. Where a Rembrandt always suggests the soul behind the skin, Freud stares at the skin itself. His people are prisoners inside their inadequate bodies in a godless and unfeeling universe.

Strangely enough, while Freud's greatness was being celebrated at the National Portrait Gallery, very similar issues of the cruelty of portraiture, and the opposed worlds of flesh and spirit, were played out this week in the less than sublime dealings between Her Majesty's government, the European court of human rights, and alleged mentor of terrorists Abu Qatada.

Abu Qatada the scholar. Photograph: AP

In his own eyes, Qatada looks like, and is, a spiritual man. His expansive waterfall of facial hair signifies his religious vocation. Tremendous beards have for thousands of years been symbols of intense male spirituality in many religions. Qatada's beard is not so very different from the outburst of chin hair sported by Pope Julius II in a 16th century portrait by Raphael, or the beards of Jewish elders in a painting by Bellini. In fact, the best way to understand how supporters of al-Qaida (of which he certainly appears to be one) who present themselves in this way imagine their own appearance, might be to look at Michelangelo's statue of Moses. Like Qatada, Michelangelo's Moses has a grandiloquent beard that dominates his face, which in the case of Moses is both gentle and furious. Moses glares wrathfully at us sinners as we enter the church that houses Michelangelo's sculpture. His beard is that of a prophet, close to God, who looks contemptuously on the deficiencies and sins of humankind.

Qatada, like Moses, is a holy man full of wrath. However, his wrath is reportedly repellent. His sermons and taped homilies are said to include calls to kill Jews and he is accused of being a spiritual mentor of the 9/11 attacks. So his photographed image has been seen this week, not spiritually as the garb of a holy man, but carnally, as gross flesh and florid hair that damns him in the eyes of parliament and public.

Abu Qatada smiles in the Mail Public Domain

He may have thought he was getting across his holiness and intellectualism when he posed in front of a bookcase. But while the Guardian used a neutral image of Qatada the scholar, the Daily Mail found a shot where his face is scrunched and lips twisted. Another set of photographs show him out shopping. When these were first published in 2008 they were scrutinised for embarrassing details, but he was only buying basic necessities, like toilet roils. The photographs of him hefting carrier bags are, however, striking because of the contrast between his ostentatious prophetic demeanour and his banal, everyday physical action of lugging stuff home from the supermarket.

Again and again, what the use of this man's image shows is the immense gap between how we picture ourselves and how we look to others. His clerical dress and beard identify him as part of a subculture of radical Islamists and within that subculture signify spiritual grace. But outside that world they mean he is a terrorist, a fanatic, a violent zealot. Pictures published this week show him with a dreamy look as he speaks on a London street and looking sleepy-eyed against a white background.

None of these photographs are caricatures. They have not, as far as I can see, been Photoshopped to distort his appearance. They turn his actual looks against him – in that cruel way portraiture has. As if concerned that he might not look fanatical enough, after all, the Telegraph also showed him alongside a hook-flaunting Abu Hamza. Yet there is no need for papers to do anything especially provocative with Qatada's pictures to make them icons of the extreme. What is intended as a holy demeanour has long since been branded as the face of violent imaginings.

Abu Qatada out shopping for toilet paper in the Telegraph Public Domain

What is disturbing is how the image of Qatada has become "proof" of his guilt. In Renaissance Florence portraits of traitors were painted by artists including Botticelli on palace walls to demonise these people, to visually damn them and exclude them from the community. Today, photographs of Islamists serve a similar purpose – and there is a bizarre collaboration between subject and starer. Qatada intentionally marks himself as a man apart, outside what he might see as the corrupt norms of modern western society. This appearance is gleefully taken by that society as a symbol of his damnable otherness. His image is used again and again as evidence of his distance from the supposed British norm. He is visually ostracised, and visually condemned.

He has not actually been convicted of a crime in any British court, in a decade of close control and surveillance. Why? Is there really no way of prosecuting him for hate crimes, given the things he is said to have said? The national rush to judge this man without trial, the parliamentary longing to deport him, perhaps to be tortured, is a betrayal of democratic values. The reliance on Qatada's appearance to condemn him is still more irrational. I have deliberately cited long-ago historical comparisons like the treatment of criminals in 15th century Florence. But there are some very obvious and humiliating 20th century parallels too for this barbaric use of images to demonise others and so justify the deprivation of their human rights.